The present invention relates to an optical head of multi-exposure type capable of giving gradation in which the circuit structure is simplified and the emission control is speeded up for exposing pixels on an image carrier to light and also relates to an image forming apparatus employing the same.
In conventional image forming apparatus in which a latent image is written on an image carrier, it is common practice to employ a light emitting diode (LED) array as writing means. In case of employing a light emitting elements such as LEDs, it is required to pay attention to the relation between the luminance (amount of light) and the life duration of each light emitting element. That is, the life duration can be increased by reducing the luminance of the light emitting element. In this case, however, there is a problem that the amount of light for exposure is insufficient to form image. When the luminance of the light emitting element is increased, enough amount of light for exposure for forming image is obtained. In this case, however, there is a problem that the life duration is shortened.
For this, the development of material for obtaining light emitting elements capable of providing large luminance and having long life duration has been encouraged. However, in the present state of affairs, it is too expensive to achieve the practical use. In this connection, a line head (optical head) of multi-exposure type in which each pixel is exposed to light by a plurality of light emitting elements. As examples of such line head of multi-exposure type, (1) Japanese Patent Unexamined Publication No. S61-182966 discloses a recording array head on which light recording elements are aligned in a plurality of lines relative to the rotational direction of a photosensitive drum. Image data are formed again and again on a same pixel by shifting the light recording elements in the direction of the lines while moving the photosensitive drum. The example (1) has an advantage that high-speed image formation is achieved even using light recording elements with low light-emitting output.
(2) Japanese Patent Unexamined Publication No. S64-26468 discloses an EL element panel composed of a group of EL elements of 20 dots (vertical)×640 dots (horizontal). The EL element group is driven for each line at the same speed of the moving speed of a photoreceptor. Accordingly, each pixel is irradiated by light of which amount is twentyfold of the amount of light of each EL element. Also in this example, the amount of light for exposure per pixel is increased, thereby achieving the speed-up of the image formation. In addition, (3) Japanese Patent Unexamined Publication No. H11-129541 discloses a print head on which LEDs are aligned in a plurality of lines. Multiple exposure is made on each pixel by moving the print head in the main scanning direction. In this example, since the multiple exposure is conducted, variations in amount of light among the respective LEDs can be equalized, thereby improving the image quality. (4) Japanese Patent Unexamined Publication No. 2000-260411 discloses an optical printer head on which plural lines of LED array chips are aligned. The gradation of each pixel can be changed among three levels by turning ON or OFF the LED array chips on each line.
The techniques disclosed in the aforementioned (1), (2) relate to monochrome image formation and have a problem that the gradation control for neutral density is impossible. Since the technique disclosed in the aforementioned (3) is of a serial type that the line head is driven, there is a problem of complexity of the driving mechanism. Since the technique disclosed in the aforementioned (4) is structured such that the LED array chips on each line are turned ON or OFF, there is a problem of complexity of the control circuit. Since the number of light emitting elements in the line head of multi-exposure type are greater than that of a line head of normal exposure type and it is necessary to control the light emitting elements synchronously with the movement of the photoreceptor, there is a problem of complexity of the control circuit for conducting the data processing and it is difficult to speed up the emission control.
Especially, in case that a line head (optical head) of multi-exposure type is employed for color image formation, the amount of data to be processed must be severalfold of that in the case of ON-OFF control because the gradation control for each pixel is sometimes required. This makes the speed-up of emission control further difficult. In case of a line head of multi-exposure type, it is required to send a large quantity of data formed by a data processor to the line head. Accordingly, the number of wires between the line head and the body of image forming apparatus is increased and it is necessary to use an interface capable of supporting the high-speed processing, thus increasing the cost.